The present invention relates to method and apparatus for drying webs and more particularly a novel method and apparatus in which a plurality of spaced parallel drying units, each having a plurality of drying emitters arranged in the cross direction of the web are operated at either a low flame or high flame state for a varying length of time within a preset duty cycle to adjust the moisture content of the web and further to provide a uniform moisture content across the web.
A paper web produced during a papermaking operation undergoes drying during the final production stage and prior to being wound upon a supply reel. Due the limitations in paper drying equipment as well as local conditions, non-uniform moisture content in the cross web direction as well as deviations in the desired moisture content are typically encountered.
One conventional technique for correcting deviations from the desired moisture content comprises the utilization of a dryer unit whose drying energy output is varies in a direct proportion to the deviation of moisture content of the web from the desired value. The disadvantages in the use of this technique reside in the fact that the valuable properties of infrared energy deteriorate drastically with a decrease in temperature. This effects energy efficiency, penetration of the radiant energy into the web, absorbtion of energy, and a unique property of increased absorbtion in higher moisture regions of the web at a specific wavelength or temperature. In addition, the ability of the radiation energy to penetrate the web varies as a function of the energy output. One technique for improving uniformity of moisture content in the cross web direction utilizes a plurality of individual emitters which are selectively operated in accordance with the moisture content of the portion of the web passing the associated emitter. One preferred embodiment of this technique is set forth in U.S. Application Ser. No. 448,619, filed Dec. 10, 1982, by the Assignee of the present invention, now U.S. Pat. No. 4,498,864 issued Feb. 12, 1985. The technique described in the aforementioned issued Patent, however, is not capable of making fine adjustments in moisture content.